THE INFLUENCE OF RISING BACKGROUND TEMPERATURE ON THE EFFECTS OF MARINE THERMAL EFFLUENTS

1. Discharged effluent waters from directly cooled power stations in temperate zones are some 8-12 degrees C warmer than intake temperatures. 2. The direct effects of thermal discharges on marine organisms fall into four categories, the mean temperature in relation to ''normal'', the absolute temperature (as it may approach lethal levels), short-term fluctuations in temperature and potential barriers to fish migration. These conditions tend to favour eurythermal species, notably those from the littoral or warmer zoogeography, while inhibiting stenotherms or cooler-water species. 3. A potential increase in background temperature would exacerbate the effects of mean and absolute temperature, although operating on a ''previously selected'' local community. While a natural temperature rise of 1 degrees C above normal can have a marked impact on even littoral species, any effluent temperature effects will be localized within a small area.